In some casinos I pay fee 0.0001 btc but in blockchain I see that they paid like example 600 satoshi fee only.May I know why?
IMO it is because the site uses a automatic system based on the current median transaction size.
I experienced it on crypto.games some time ago and I asked about it as well, and here is the information provided by crypto.games:
Example: I was charged 0.0005 BTC fees for my withdrawal, but the bitcoin transaction sent used a lower fee, why does this happen?
The withdrawal fee charged to a user issuing a withdrawal is the fee for a transaction with the size of 500 bytes (which is the current median transaction size according to tradeblock), in this exemplary case, 0.0005 BTC. The actual transaction sent uses a per kB fee that’s twice the size of your charged withdrawal fee (in this example, 0.0005 BTC are charged, and 0.001 BTC per kB are used). However, if the transaction is smaller than the average, the total fee will be lower accordingly. Likewise, if the transaction is bigger than the average, the total fee will be higher accordingly. In either case, the fee charged to the user will be the same, 0.0005 BTC, as it’s not possible to estimate the size a withdrawal transaction will have before the withdrawal is requested. In any way, the fee you are being charged is always the fee that is shown on the slider (“withdrawal fee”), where you select the fees, and the amount you receive is always the amount shown in the withdrawal popup (“Withdrawal amount after fees”).
Perhaps the casino where you play uses the same system as crypto.games, that's why the fee is for the withdrawal is not the same as what you paid.
I do believe most casinos uses similar system as there is no need for them to pay higher fee while the current network does not requires them to pay higher.